1. Field of Use
This disclosure relates to preprocessing signals to enhance audibility in a magnetic resonance imaging (MRI) and fMRI systems; and in particular, relates to preprocessing musical signals through the exploitation of spectral masking in the human auditory system to enhance audibility.
2. Description of Prior Art (Background)
In Magnetic Resonance Imaging (MRI) devices, a patient lying in the bore of the main magnet is subjected to considerable noise levels that are created in the bore due to gradient switching, helium pump operation, ventilation equipment, etc. In order to protect the ears of the patient, circumaural headphones or earplugs are typically provided. Additionally, an operator of the MRI device may communicate with the patient via such headphones.
However, headphones do not provide sufficient attenuation of the ambient noise, and considerable noise reaches the patient's ears through tissue and bone conduction. Even with the best circumaural headphones, patients are subject to considerable noise levels, largely originating from the gradient coil system in the MRI device.
In addition, patients experience considerable discomfort (e.g., physical, psychological, emotional, etc.) when undergoing an MRI scan. For instance, the patient may be experiencing physical pain due to an illness, psychological or emotional pain or worry related to the illness, claustrophobia due to the cramped space within the bore of the MRI device, etc. The loud repetitive noise of the gradient coils can exacerbate these discomforts, increasing patient stress. Further, functional MRI (fMRI) related noise can be a serious impediment in the case of fMRI studies of the brains responses to stimuli; especially studies involving musical stimuli where audibility is critical.
Currently, widely-used methods attempt to reduce the maximum sound pressure level and do this by constructional sound deadening methods, for example, active phase cancellation; or, via a smaller gradient load. Another way is to reduce the sound pressure level at the ear, for example with headphones or ear plugs. Methods for sound extinction in the vicinity of the ear using interference are hardly ever implemented on account of the strong magnetic fields and the restricted space available. Headphones or earplugs also have only a very limited protective function since the loud knocking sounds can be transmitted not only via the auditory canal but also via the cranial bone into the inner ear and can thus simply thus not be filtered out just like that. Constructional sound deadening methods such as a heavier encapsulation of the coils and leads have only proved effective to a limited extent and reducing the sound by imposing less of a load on the gradient coils results in lower quality imaging.
In light of the above, it will be appreciated that there exists a need for masking noise generated by medical imaging devices such as MRI systems.